1. Field of the Invention
The present invention relates to a hologram reproduction apparatus which reproduces two-dimensionally modulated digital data.
2. Description of the Related Art
There is known a hologram recording technique which records information to be recorded on a hologram recording medium (hereinafter simply referred to as a “recording medium”) as interference fringes. One method for this technique uses the information to be recorded to spatially modulate a light from a light source to generate an object light. The apparatus irradiates the recording medium with an object light and a reference light. The object light and the reference light generate interference fringes on the recording medium, and the interference fringes are recorded in a recording layer of the recording medium. On the other hand, for reproduction, the interference fringes recorded on the recording medium are irradiated only with the reference light. A two-dimensional sensor detects a detected light from the recording medium to reproduce the recording information. An example of the above-mentioned hologram recording and reproduction apparatus is disclosed in Japanese Patent Application Laid-open under No. 11-16374.
There is known such a technique that a gain is adjusted in order to make data suitable for A/D conversion before converting (A/D-converting), into digital data, analog data incorporated into an apparatus and the like, which is called “gain control”. The technique is disclosed in Japanese Patent Application Laid-open under No. 9-288309. This document discloses such a technique subjected to control of a natural image that a peak value of the quantized digital data is detected, and that the gain is set to be high when the peak value is smaller than a target peak value (a half of a maximum value of a dynamic range of the A/D conversion) and the gain is set to be low when the peak value is larger than the target peak value.
The gain control disclosed in the above-mentioned document is effective to the natural image. However, in a two-valued image which is two-dimensionally modulated and which is used in the above-mentioned hologram recording technique, a reproduction S/N after the A/D conversion is sometimes decreased. This is because the dynamic range of the A/D conversion cannot sometimes be effectively used to the two-valued image which is two-dimensionally modulated, if the target peak value of the pixel is determined to be the half of the maximum value of the dynamic range of the A/D conversion as described above.